1. Field of the Invention
The present invention relates to an apparatus for obtaining biometric information such as fingerprints, palm prints, and blood vessel patterns. The invention relates particularly to a biometric information obtaining apparatus which obtains information needed for user verification from partial images (fingerprint images) that are successively obtained while a finger is moving relative to the sensor surface (image capturing surface) of a sweep-type fingerprint sensor. Further, the present invention relates to a verification apparatus on which user verification is performed using biometric information such as fingerprints, palm prints, and blood vessel patterns.
2. Description of the Related Art
With recent improvement in functionality, small-sized information equipment, such as mobile phones and PDAs (Personal Digital Assistants), is capable of connecting to communication networks and storing a great amount of personal information, thereby increasing the demand for improvement in security of the equipment.
To realize the security of the equipment, pass words and ID (Identification) cards have been used for user verification. However, since such passwords and ID cards are easily stolen, more reliable user verification (confirmation that an object user is an authorized user who has been registered beforehand) has been strongly desired. To meet this demand, use of biometric user verification, which is highly reliable in verification, is conceivable. In particular, fingerprints are convenient for users.
When a fingerprint is used as biometric information for user verification, a fingerprint sensor of an electrostatic capacity type or of an optical type is used to obtain image information of a fingerprint (a pattern formed of fingerprint ridges that contact the image-capturing surface of the fingerprint sensor and valley lines that do not contact the image-capturing surface) from a fingertip of a user. Characteristic information (for example, position information of bifurcations and endpoints) is extracted from the foreground (for example, fingerprint ridge images) of the fingerprint image, and the extracted characteristic information is verified against the user's characteristic information that has been registered beforehand, thereby carrying out authentication of the user.
Fingerprint sensors commonly used have a sensor surface (image-capturing surface) larger than a human fingertip. However, recent fingerprint sensors are given a sensor surface that is smaller than a human fingertip, to make it possible to provide such fingerprint sensors to small-sized information equipment such as mobile phones and PDAs. Partial images of a fingerprint, which are successively obtained through such a small-sized sensor surface, are combined to obtain the whole fingerprint image.
A sweep-type fingerprint sensor is one of such fingerprint sensors with a small-sized sensor surface. A sweep-type fingerprint sensor has a small-sized rectangular image-capturing surface (sensor surface/image capturing surface) which is sufficiently shorter in length than a fingertip. The fingerprint sensor successively obtains partial images of a fingerprint, while an object fingertip is moving relative to the image-capturing surface, or while the image-capturing surface is moving relative to the fingertip, and the whole image of the fingerprint is reproduced from the partial images. Here, such movement of a finger relative to the image-capturing surface is called “sweep”. Fingerprint minutiae (ridge bifurcations and ridge endpoints) are extracted/generated from the thus reproduced fingerprint image, and user verification is performed based on the minutiae information.
Techniques of reproducing the whole fingerprint image from fingerprint partial images are disclosed in, for example, Japanese Patent Application Publication No. HEI 8-263631 and Japanese Patent Application Publication No. 2001-155137. In the techniques disclosed in these patent applications, a relative position between a finger and a sensor in a direction orthogonal to the lengthwise direction of the sensor is obtained to reproduce the whole image of the finger. In techniques disclosed in U.S. Pat. Nos. 6,289,114 and 6,317,508, overlap between images successively obtained is measured, and the whole finger image is reproduced based on the overlap.
However, since an image obtained by such a sweep-type fingerprint sensor and then reproduced is distorted due to varying factors, it is sometimes difficult to compare the fingerprint data obtained with a sweep-type fingerprint sensor and fingerprint data registered beforehand.
For example, fingerprint sensors in which a fingerprint image is obtained while a finger is moving on a sensor surface in contact with the sensor surface have the following problems. As will be described later with reference to FIG. 5 through FIG. 7, and FIG. 27 through FIG. 31, as a finger, which is flexible and three-dimensional, is pressed against a plane sensor surface, the finger is deformed, whereby a large distortion is caused in a fingerprint image. Further, since a portion of a finger sometimes adheres to the sensor surface due to friction between the sensor surface and the finger, the finger is deformed, whereby a large distortion is caused in a resulting fingerprint image. Such deformation depends on the direction in which a finger sweeps.
Further, as shown in FIG. 19 through FIG. 26, if an image-capturing device, which obtains an image of a fingerprint, has any detection delay, captured images are distorted irrespective of distortion of the finger itself. Taking an example of a sensor in which information is detected by one scanning line, and information is detected by the adjacent scanning line after elapse of a specific delay, (see FIG. 25), if relative position information between a finger and the sensor is greatly changed over time, an aspect ratio of a captured fingerprint image is changed, thereby causing an extractive/contractive distortion (see FIG. 21 through FIG. 24). Further, in a sensor in which detection delay appears for each cell (see FIG. 26), a skew distortion occurs (see FIG. 25 and FIG. 26).
Such distortion deteriorates the reproducibility of a reproduced image. Further, when partial images which are distorted in different ways are connected, patterns (images of sebum or sweat secreted from a finger, or images of flaws on the sensor surface, or an image due to a defect in a detector device) that do not actually exist in a fingerprint could appear. That is, characteristic information obtained from a user with a fingerprint sensor is significantly different from the user's registered characteristic information because of the distortion described above or the appearance of patterns that do not exist in a fingerprint. Thus, a false verification result could be obtained even if an object user to be verified is an authorized user, so that verification ability, that is, verification reliability, is deteriorated.
Further, in the techniques disclosed in the above Japanese patent applications and in the above U.S. patents, the following problems are also caused. In order to reproduce the whole fingerprint image from partial images obtained, a memory with a capacity enough to store the whole image of a finger needs to be provided, and the process time is also increased. Thus, the above techniques are not applicable to small-sized information equipment.
In view of this, the object of the present invention is to perform correction on characteristic information extracted from each partial image, thereby making it possible to obtain undistorted characteristic information using a memory with a small capacity. As a result, user verification is performed based on high-quality characteristic information, so that verification ability, that is, verification reliability, and user convenience are improved.